The present invention relates to an improved double wrap brake band and the method of making same for use in the automatic transmission of an automotive vehicle. In a conventional automatic transmission of the torque converter type, a driven shaft extends from the torque converter into the transmission to a unit having one or more clutch drums around which are placed double wrap brake bands; and more specifically, the double wrap bands are used for the 1-2 unit and for the reverse unit. It is the function of the brake bands under suitable hydraulic controls to effect gear change and control of the direction of rotation of the output or drive shaft from the transmission.
Presently manufactured double wrap brake bands are formed from a relatively thick metal strap and a forging or casting forming ears and a transverse tie for the band; the ears being engaged by the struts and a hydraulic piston in the transmission to actuate the band. The forging or casting is located in the space between the ends of the arcuate strap and the edges of the casting and strap are butt welded together. As seen in U.S. Pat. No. 2,867,898, once the bracket is welded to the strap, the assembly is expanded to size and shape, the bracket is transversely broached to form the transverse tie and ears separated by parallel slots and to form "V" slots in the ears to accommodate the struts, openings are pierced in the bottom wall of the broached slots, the interior surface of the assembly is shot peened and the friction lining is bonded to the surface. After bonding, openings are pierced in the friction material, the interior surface is bored, the friction lining is circumferentially grooved, and two circumferential slots are formed through the assembly, except the transverse tie, by sawing through the strap and lining to produce the three radially expansible bands.
The double wrap bands made by the above enumerated method have certain disadvantages in operation. The internal stesses created in the bands during manufacture will distort the bands so that they are not round in the released position. Thus, when assembled in an automatic transmission, there will be a certain amount of drag between the bands and its associated drum. Also, when the bands are actuated to engage the drum, a lack of concentricity of the outer and middle bands results in uneven engagement of the band. The double wrap brake band of the present invention obviates the disadvantages of the previously used bands.